1. Field of the Invention
This invention relates to an antenna for satellites to communicate with ground stations and, more particularly, to an antenna system for satellites incorporating an antenna array and a Butler matrix for producing a communicating beam steerable by varying a carrier frequency of the beam.
2. Description of the Related Art
Satellites are now employed for providing communication, such as telephone in land mobile service, between distant points on the surface of the earth. One embodiment of such a system is of considerable interest, namely, an embodiment wherein the satellite travels in a geostationary orbit about the earth. For example, the satellite may be located at a fixed position above the United States. The satellite would carry an antenna having a sufficient beam width in the north-south direction and in the east-west direction to permit the reception and transmission of communication signals between any two points in the United States. The beam width in the north-south direction can be enlarged to include both United States and Canada, if desired. A beam width of approximately 4.5.degree. in the north-south direction is sufficient to cover both Canada and the United States. The beam width in the east-west direction should be approximately 8.degree. to provide the desired coverage. A problem arises in that the use of an antenna having the foregoing beam width in the north-south and east-west directions has less signal gain than is desired. This necessitates larger power amplifiers for driving radiating elements of the antenna.
In previous satellite communication systems, such a wide beam width antenna has employed at least two overlapping beams to provide the coverage. The generation of such beams with a desired overlap until very recently required the use of separate large reflectors each having a diameter of about 16 feet. In the construction of communication satellites, however, it is desirable to reduce physical sizes, weights, and power requirements to facilitate the construction and launching of such satellites.
In commonly assigned, copending U.S. patent application Ser. No. 782,770 filed Oct. 1, 1985 in the name of H. A. Rosen and entitled STEERED-BEAM SATELLITE COMMUNICATION SYSTEM, which is hereby incorporated by reference, there is disclosed a system for communicating via satellite between ground stations. The system comprises a set of ground stations spaced apart along an arc of the earth's surface and a satellite positioned above the earth in view of the arc. An array of radiating elements is deployed on the satellite, and a frequency responsive beam former connected to the radiating elements is provided for forming a beam of electromagnetic radiation. The beam is steerable in response to a carrier frequency of the radiation to intercept individual ones of the stations in seriatim. The frequencies of an up-link carrier and of a down-link carrier respectively associated with respective ones of the ground stations vary monotonically with position along the arc to permit automatic positioning of a beam from the satellite to a ground stations upon energization of a carrier frequency assigned to the ground station.
So that the present invention may be better understood, the satellite communication system disclosed and claimed in the aforementioned application will now be discussed in some detail by reference to FIGS. 1 through 5. As shown in FIGS. 1 and 2, the satellite 24 employs a simplified antenna structure 30 comprised of two confocal parabolic reflectors, one of which is a large main reflector 32 and one of which is a small subreflector 34, and a 4.times.2 array 40 of eight radiating elements 42, all of which are supported by a frame 44. A front view of the array 40 is shown in FIG. 2. The array 40 of radiators 42 is rigidly secured in front of the subreflector 34, and with the subreflector is located within the satellite 24. The main reflector 32 is substantially larger than the subreflector 34, and due to the larger size, is folded during launch, and is subsequently unfurled when the satellite or spacecraft 24 has been placed in orbit. Upon being unfurled it extends outside of the satellite 24 as shown. Also shown in FIG. 1 within the frame 44 is other spacecraft equipment such as rocket engines and fuel tanks, thereby to demonstrate that the antenna system 30 can be easily carried by the satellite 24.
The arrangement of the components of the antenna system 30 provides a significant reduction in weight and complexity for a satellite antenna over that which has been employed before. This is accomplished by fabricating the main reflector 32 and the subreflector 34 with parabolic reflecting surfaces, the two surfaces being oriented as a set of confocal parabolas having a common focal plane or point 48. Such configuration of reflecting surfaces in an antenna is described in C. Dragone and M. Gans, "Imaging Reflector Arrangements to Form a Scanning Beam Using a Small Array", Bell System Technical Journal, Vol. 58, No. 2, (Feb. 1979), pp. 501-515. The configuration provides a magnification of the effective aperture of an array of radiating elements. In the preferred configurations as shown in FIGS. 1 and 2, the magnification factor is 4.7. The eight radiating elements 42 of the array 40 represent a substantial reduction in complexity of the antenna since, if a direct radiator of similar sized elements had been employed, a total of 155 radiating elements would have been needed to give the same antenna performance. As shown in FIG. 3, a hexagonally arranged antenna array 50 of seven primary radiators 52 may be used if desired in place of the 4.times.2 array of radiators mentioned above. The array 50 of feed elements 52 may be employed for both up-link and down-link communications.
FIG. 4 illustrates two exemplary spot beams 56, 58 produced by the satellite 24 (not shown in FIG. 4) in geosynchronous orbit above the earth 60. Spot beam 56 extends substantially along the eastern coast of the United States 62 and Canada 64, while spot beam 58 extends substantially along the western coast of the United States 62 and Canada 64. The satellite transmits and receives information-carrying radiation to and from ground stations located within regions of the earth's surface encompassed by the respective first and second spot beams 56, 58. The coverage patterns of the respective spot beams 56, 58 preferably are selected such that frequency bands available for communications are concentrated in regions of the surface of the earth 60 where the largest communications capacity is necessary, to optimize antenna gain usage by substantially limiting the amount of antenna gain which is incident upon regions wherein relatively little communications capacity is necessary, such as in sparsely populated regions.
The antenna system of satellite 24 provides a one-dimensional beam scan (which may be considered to be a continuum of virtual spot beams) across the surface of the earth 60. Such a scan can be directed along an arc of the earth's surface such as a longitudinal or a latitude, or an arc included relative to a latitude. The scanning can be accomplished most efficiently for the geography depicted in FIG. 4 by scanning in the east-west direction providing a scan path which follows an arc of a great circle of the earth. The scanning is preferably emplemented by using fixed delays (as will be described hereinafter) among radiating elements of the antenna system and by employing different frequencies for different geographical locations on the surface of the earth. Thereby, the scanning is accomplished by variation of the frequency of the radiation for each position of the beam can (i.e., for each virtual beam), and in addition, a plurality (not shown) of the beams can be generated simultaneously by the provision of different frequencies of electromagnetic radiation in each of the beams. By use of this virtual beam technicque, users at any point within the coverage of the beam scan are close to the center of one of the virtual beams. Therefore, users will typically receive 2 or 3 dB more power than they would from a comparable satellite using fixed beams.
To minimize the required electromagnetic power and provide for simplicity of antenna structure, the preferred antenna system provides beams with a generally circular cross section and a width of 4.5.degree., by use of the hexagonal array 50 of radiating elements 52 as shown in FIG. 3. The elements 52 preferably are cup dipole feed horns one wavelength in diameter.
As an example of its use, the satellite communications system may be designated for land mobile telephone service, sometimes referred to as the Mobile Satellite (MSAT) system. Two frequency bands are assigned for such service: 866-870 MHz for the down-link band and 821-825 MHz for the up-link band. The 4 MHz width of each of these bands may be subdivided into approximately 1000 frequency slots which are individually assignable to individual ground stations on the surface of the earth 60 for companded single sideband voice communication. If the stations were uniformly positioned from east to west, with each station being at a different longitude, approximately twelve assignable channels comprising an up-link and a down-link would be available within a scan angle of approximately 0.1 degree.
Since the channels would be uniformly spaced apart in frequency, a beam would be uniformly stepped in the east-west direction as the down-link (or up-link) frequency was shifted from one channel to the next channel. In other words, the operating frequency of the ground station is preferably selected to match the frequency of a beam directed from the satellite to the ground station. For a uniform distribution of the stations in the east-west direction, the beam could be centered with respect to the east-west component thereof, upon each of the stations. However, as a practical matter, the stations tend to be clustered in various geographic areas of the United States 62 and Canada 64 providing a nonuniform distribution of the stations along the east-west scanning path of the beam. Consequently, a peak signal amplitude cannot be obtained for all of the stations.
By way of example, assuming that 25 ground stations are located within a scan angle of 0.1.degree., the corresponding reduction from peak signal amplitude is less than 0.01 dB (decibels). This represents a significant improvement over previously available satellite communication systems employing separate fixed beams wherein the average loss in signal gain relative to peak signal gain in the east-west direction was approximately 0.8 dB. As noted above, such previous satellite communication systems employed antenna systems having a plurality of large antenna reflectors, measuring approximately 16 feet in diameter, while the antenna system described in the aforementioned patent application requires only one such large reflector and a much smaller confocal subreflector as will be described hereinafter. Thus, the disclosed system provides for improved uniformity of signal gain with a simplified mechanical structure of the antenna system.
FIG. 5 presents a diagram useful in explaining the frequency scanning operation of the antenna system. A set of four radiating elements 42 are arranged side by side along a straight line, and face an outgoing wavefront 66 of electromagnetic radiation. The angle of incidence of the wavefront or beam scan angle is measured relative to a normal 68 to the array 40 of elements 42. A frequency scan is generated in a planar array antenna by introduction of a progressive time delay into the array. The progressive time delay provides for a difference in the phases of signals excited by adjacent ones of the elements 42 such that the phase difference is proportional to the frequency of the radiated signals. This explanation of the operation assumes an outgoing wavefront, it being understood that the operationg of the array of elements 42 is reciprocal so that the explanation applies equally well to an incoming wavefront. The relationship of scan angle to frequency, element spacing, and time delay is given by the following equations: EQU 2.pi.D sin .theta.=.DELTA..PSI.=2.pi.f.DELTA.T (1)
therefore, EQU sin .theta.=.lambda./D f.DELTA.T (2)
wherein:
D=spacing between elements, PA1 .theta.=beam scan angle, PA1 .lambda.=wavelength of radiation, PA1 .DELTA..PSI.=phase increment between adjacent elements, PA1 f=frequency relative to band center, and PA1 .DELTA.T=time delay increment between adjacent elements.
The radiating elements 42 are energized via a source 70 of microwave energy and a series of delay units 72 coupled to the source 70. Each of the delay units 72 provides the time delay increment referred to above in Equations (1) and (2). The source 70 is connected directly to an element 42 at the left side of the arry while the next element 42 is connected by one of the delay units 72 to the source 70. The signals applied by the source 70 to the third and the fourth of the elements 42 are delayed, respectively, by two and three delay increments of the delay units 72. This provides the linear phase relationship to provide the scan angle for the outgoing wavefront 66. The phase increment between two adjacent ones of the radiators 42 is proportional to the product of the frequency of the radiation and the delay increment. When this product is equal to 360.degree., the wavefront propagates in a direction normal to the array of elements 42. Increasing values of frequency produce greater phase shift to direct the wavefront to the right of the normal 68 as shown in FIG. 5, while decreasing amounts of frequency produce less phase shift and drive the wavefront to the left of the normal. Accordingly, the wavefront can be scanned symmetrically about the array of elements 42.
The aforementioned application also discloses that for the case of the foregoing up-link and down-link frequency bands, and for the case of the radiating elements 42 having a diameter of approximately one wavelength, a suitable value of differential delay, as provided by the delay units 72 of FIG. 5 is 185 nanoseconds for the case of substantially uniform distribution of ground stations on the surface of the earth 60. To provide the east-west coverage of 8.degree., the up-link and the down-link beams are scanned through an arc from -4.degree. to +4.degree.. In view of the magnification factor of 4.7, the scan angle of the array 40 of radiating elements 42 must be enlarged by the same magnifying factor, 4.7, from that of the output scan from the main reflector 32. Therefore, the beam produced by the radiating elements 42 must be scanned through an arc of 18.8.degree. to either side of a normal to the array 40. The foregoing value of differential delay, namely, 185 nanoseconds, provides the 18.8.degree. scan to either side of the normal to the array 40. In th ideal situation of uniformly distributed ground stations between the East Coast and the West Coast of the United States and Canada, the number of channels per degree has a constant value of 1000/8=125.
In the situation wherein the differential delays provided by the delay units 72 are independent of frequency, then an optimal direction of the scanned beam is obtained for the ideal situation of uniform distribution of ground stations. In the more likely situation of a nonuniform distribution of ground stations, the scanned beam may be displaced slightly from its designated ground station. As has been noted above, such a beam-pointed inaccuracy reduces the signal level by less than 0.01 decibels for a beam-pointing error of 0.1 degree.
The aforementioned patent application discloses that the scanning can be adapted to accommodate the foregoing nonuniformity in ground-station distribution by introducing a frequency responsive component to the differential delay. It gives an example of nonuniform distribution where the differential delay between columns of the array 40 of radiating elements 42 (see FIG. 4) should vary, at least for the forming of the down-link beams, between 262 nanoseconds at the low frequency end of the transmission band to 131 nanoseconds in the high frequency end of the transmission band. Other values of delay may be employed in the beam forming operation of up-link beams provided by the receiver of the antenna system (30).
The values of delay used in the different frequency bands, namely, the up-link and down-link frequency bands, are inversely proportional to the center frequencies of these bands as is apparent from Equations (1) and (2). A reduction in the differential delay results in a reduced amount of phase shift between successive beams with a corresponding reduction in displacement of beam position on the surface of the earth 60 from one channel to the next channel. Thereby, the beam can be more accurately positioned in a region of high density of ground stations. In a corresponding fashion, an increase in the differential delay results in increased movement of the beam as the frequency is shifted from one channel to the next channel, thus accommodating positions of the beam to a less dense distribution of ground stations. The channel number corresponds to a specific frequency in either the up-link or th down-link band. With respect to the positioning of ground stations along an arc of a great circle of the earth 60, as disclosed with reference to FIG. 4, it is seen that the frequencies selected for the various stations vary monotonically with position along the foregoing arc.
In view of the foregoing description, it is seen that the above described communication system provides two-way communications between ground stations and a geosynchronous satellite. The assignment of specific frequencies to respective ones of the ground stations, in combination with frequency scanning of both up-link and down-link beams of the satellite (24), permits a simplification in the circuitry of the system. In addition, the use of the two confocal parabolic reflectors provides a multiplicative factor which reduces the number of elements required in the array of radiating elements. The use of a scanned beam also reduces the physical size of the antenna system by reducing the number of reflectors, resulting in a lighter weight, more efficient satellite communications system.
It has been found that certain technical impediments exist to the commercial implementation of the above described confocal reflector system. Due to spacecraft size limitations the subreflector 34 cannot be constructed large enough (in terms of wavelengths) to perform with acceptable efficiency. These size limitations also restrict the size of the main reflector and the focal lengths that may be used in the confocal arrangement.
It would be desirable, in order to achieve a further weight-saving and simplification of the aforementioned satellite communications system, to eliminate the subreflector altogether while still utilizing a relatively low number of radiating elements. It would also be very advantageous to be able to combine the power of output signals from several individual amplifiers operated in parallel into an individual one or small group of the radiating elements so as to produce a stronger spot beam in any given location along the area of the earth being swept by the scanning beam. It would further be desirable to use as many elements as possible as common elements in an antenna system for the transmitter antenna system and receiver antenna system of a communications satellite so as to save weight, space and cost. The present invention is directed to achieving these and other desirable objects.